Device for the reproduction of sound



June 24, 1958 BOLESLAV DEVICE FORTHE REPRODUCTION OF SOUND Filed Jan. 3, 1955 FIG. 3

FIG. 7

INVENTOR. uZ/e 750/0; /a' 1/ WWW DEVICE FOR THE nnPnonUcTroN OF SOUND Ales Boleslav, Prague, Czechoslovakia, assignor to Tesla,

narodni podnik, Prague, Czechoslovakia, a corporation of Czechoslovakia Application January 3, 1955, Serial No. 479,635

Claims priority, application Czechoslovakia Eanuary 20, 1954 It) Claims. (Cl. Isl-31) This invention relates to an acoustic cabinet for loudspeakers.

The size of known devices for the reproduction of sound, particularly for good reproduction in the range of low frequencies, is large and this results in various drawbacks. For example, their production costs and weight are high and such devices are bulky. It is necessary to remove standing waves which cause a r1pple on the frequency characteristic and bad transmission of 1mpulses. It is also necessary to eliminate radiation caused by vibration of the cabinet walls.

It has already been proposed to reduce the size of an enclosed loudspeaker cabinet by loading the back of the diaphragm of the loudspeaker mounted within the cabinet with an acoustic impedance which consists of an acoustic mass in parallel with an impedance the resistance component of which predominates within the resonance range of the system, the desired frequency characteristic and the correct damping of the diaphragm being obtained by a suitable choice of the magnitude of the loading impedance. In comparison with existing devices of this type, it has been possible to obtain in this way a reduction of the size of the loudspeaker cabinet and to improve the electroacoustic properties thereof.

It is an object of this invention to further simplify and improve such devices, especially in the case of small and medium sized directly radiating loudspeakers and coaxial loudspeakers, where the high frequency system is provided with an exponential horn the output section of which is much smaller than the surface of the diaphragm for the reproduction of low frequencies.

The device for the reproduction of sound according to this invention is characterised by the fact that the front of a directly radiating loudspeaker, mounted within a cabinet, is loaded, in addition to its own radiation impedance, also with an acoustic impedance consisting of an auxiliary circuit comprising parallel components, one of which constitutes in the range of low frequencies a predominantly acoustic resistance and the other a predominantly acoustic mass, the desired frequency characteristic being obtained by a suitable choice of both components.

The invention will be ,best understood from the following detailed description of illustrative embodiments to be read in conjunction with the accompanying drawings in which Fig. 1 is a vertical sectional ing to the invention;

Fig. 2 is a front elevational view of the device of Fig. 1;

Fig. 3 is a vertical sectional view of another embodiment of the invention employing a coaxial loudspeaker;

Fig. 4 is a front elevational view of the device of Fig. 3;

Fig. 5 is a front elevational view of an auxiliary loading circuit according to the invention consisting of an acoustic resistance and an acoustic mass;

Fig. 6 is a vertical sectional view of the loading circuit of Fig. 5;

View of a device accord- States Patent 0 Fig. 7 is a vertical sectional view of another embodiment of the invention; and

Fig. 8 is a front elevational view of the device of Fig. 7.

Referring now to Figs. 1 and 2, 1 designates an enclosed loudspeaker cabinet, 2 is a loudspeaker system, 3 is an acoustic resistance, and 4 is an opening acting as acoustic mass. The arrangement of the acoustic resistance 3 and the acoustic mass 4 can be seen from Figs. 5 and 6.

The acoustic resistance 3 consists of a rigid or reinforced disk of wire-net, or a perforated metal sheet on which there is cemented a material acting as acoustic resistance. The opening 4 in the center of the disk acts as acoustic mass. The diameter of the disk which carries the acoustic resistance material is the same as the diameter of the mouth of the loudspeaker. The disk is pressed by the rim of the loudspeaker against the inner face of the front wall of the loudspeaker cabinet which is provided with an opening for the loudspeaker.

It is, of course, also possible to clamp the acoustic resistance material between two rigid and air permeable disks (of Wire-net or perforated metal sheet), or to employ a sufiiciently rigid resistance material which practically will not vibrate during operation of the device.

In order to secure good reproduction of higher frequencies which are mainly radiated from the center of the loudspeaker diaphragm, the opening 4a should lie in the middle of the disk 3a. This is particularly important with a coaxial loudspeaker 2a, as illustrated in Figs. 3 and 4.

In this case, it is necessary to choose the size of the opening 4a so that the difference between its total surface and the surface of the mouth of the horn of the high frequency system has a value which corresponds to the necessary mass of air in the acoustic circuit. Since the mouth of the horn of the high-frequency system lies in the center of the opening 4a, as can be seen in Fig. 4, the radiation of high frequencies remains unaffected.

The size of the opening 4 or 40, and hence also the size of the acoustic mass, should be chosen so that the impedance of the loudspeaker cabinet 1 or la determined by the acoustical capacitance of the volume of this cabinet compensates partly or completely the impedance of the acoustical mass in the vicinity of the resonance frequency of the employed loudspeaker. At frequencies above the resonance frequency of the loudspeaker 2 or 2a, the acoustic resistance 3 or So begins to shunt the impedance of the acoustic mass 4. If the acoustic resistance is chosen so that, at medium frequencies, it is substantially smaller than the impedance of the vibrating system, there is practically no reduction in the average level of the radiated acoustic signal. The arrangements according to Figs. 1 and 2 and Figs. 3 and 4 give a very good frequency characteristic at small production costs.

Figs. 7 and 8 show another modification of the invention. In this case, the acoustic mass which has been constituted in the previous embodiments by the mass of the air within the opening 4 or 4a is now constituted by an auxiliary diaphragm 4 which is disposed in front of the low frequency diaphragm of the loudspeaker 2b in the cabinet 1b and as close thereto as possible. The natural resonance of the auxiliary diaphragm 4' must be lower than the resonance of the loudspeaker system at least by one half.

This auxiliary diaphragm is provided with openings 3' which are either filled or covered with a material acting as acoustic resistance. It is of course possible to make the whole auxiliary diaphragm 4 of an air permeable material which acts as an acoustic resistance.

This modification operates in the same manner as the first described embodiments.

In all the above described embodiments of the invention,v the acoustic circuit, that is, the disk provided with an acoustic resistance and with an opening acting as acoustic mass, or the auxiliary diaphragm according to Figs. 7 and 8, can be made to form a part of the loudspeaker cabinet.

What I claim is:

1. A device for the reproduction of sound comprising a cabinet having a loudspeaker opening at the front.

thereof and being otherwise closed, a loudspeaker within said cabinet and extending across said opening in a directly radiating position, said loudspeaker having a diaphragm and a system for vibrating the latter, and means defining an acoustic impedance loadingthe front side of said loudspeaker diaphragm and consisting of two parallel elements, one of said elements being predominantly an acoustic resistance within the range of low frequencies and the other of said elements being predominantly an acoustic mass Within the range of low frequencies, the values of said acoustic mass and acoustic resistance being related so that, in the resonance range of the loudspeaker, the loading acoustic impedance defined by said acoustic mass and acoustic resistance compensates the acoustic impedance of said closed cabinet and, in the range of medium frequencies, the value of the acoustic resistance is lower than the value of the reactance of said loudspeaker vibrating system, so that there is a minimum reduction in the average level of the acoustic signal radiated by said loudspeaker.

2. A device for the reproduction of sound. comprising a cabinet having a loudspeaker opening at the front thereof and being otherwise closed, a loudspeaker within said cabinet and extending across said opening in adirectly radiating position to close said opening, said loudspeaker having a diaphragm and a system for vibrating the latter, and means loading the front side of said diaphragm with an acoustic impedance and consisting of a member of acoustic resistance material which-represents predominantly an acoustic resistance within the range of low frequencies, said member having an opening with the air within the latter defining predominantly an acoustic mass in parallel with said acoustic resistance within said range of low frequencies, the values of said acoustic mass and acoustic resistance being such that, in the resonance range of said loudspeaker, said loading acoustic impedance resulting from said acoustic mass and resistance substantially compensates the acoustic impedance of said closed cabinet and, in the range it of medium frequencies, the value of said acoustic resistance is lower than the impedance of said vibrating system thereby to minimize reduction of the average level of the acoustic signal radiated by the loudspeaker.

3. A device as in claim 2; further comprising a rigid air-permeable base of wire net to which said member of acoustic resistance material is attached.

4. A device as in claim 2; further comprising a perforated metal sheet forming a rigid air-permeable base to which said member of acoustic resistance material is attached.

5. A device as in claim 2; further comprising two rigid, spaced apart air-permeable discs at opposite sides of said member of acoustic resistance material to rigidly support the latter.

6. A device as in claim 2; wherein said acoustic resistance material is rigid and self-supporting.

7. A device as in claim 2; wherein said member of acoustic resistance material is in the form of a disc having the same outer diameter as that of said diaphragm with the outer periphery of the latter pressing said member against the inner surface of said front of the cabinet around said loudspeaker opening of the latter.

8. A device as in claim 1; wherein said means defining a loading acoustic impedance includes an auxiliary diaphragm disposed in front of said loudspeaker diaphragm and close to the latter, said auxiliary diaphragm having openings therein, and acoustic resistance material extending across said openings of the auxiliary diaphragm.

9. A device as in claim 1; wherein said means defining a loading acoustic impedance includes anauxiliary diaphragm in front of said loudspeaker diaphragm and close to the latter, said auxiliary diaphragm being of an air-permeable material acting as an acoustic resistance.

10. A device as in claim 1; wherein said means'defining a loading acoustic impedance forms a part of said cabinet atsaid loudspeaker opening of the latter.

References (Iited in the file of this patent V UNITED STATES PATENTS 

